A hot runner injection molding apparatus may include a valve pin gating mechanism at a mold gate of a mold cavity to control the flow of a melt stream into the mold cavity. Typically, the valve pin is disposed within a melt channel of the nozzle to slidably reciprocate between an open position, in which the valve pin is removed from the mold gate, and a closed position, in which the valve pin is seated in the mold gate. In order to provide a good seal at the mold gate, both a tip portion of the valve pin and the corresponding sealing surface of the mold gate must typically be machined to very close tolerances.
Various factors may cause the valve pin tip to become misaligned as it approaches the mold gate. For example, the nozzle in which the valve pin moves may be misaligned with the mold gate. Also, thermal expansion and contraction of various components of the injection molding apparatus, which takes place repeatedly during an injection molding run, may cause components to shift, ultimately resulting in misalignment of the nozzle and, consequently, the valve pin with the mold gate. Another factor may be non-homogeneity in the melt that can cause the melt to exert uneven fluid pressure on the valve pin body pushing the tip, i.e., the sealing end, of the valve pin out of alignment with the mold gate.
When a misaligned valve pin is moved to close a mold gate, the valve pin may collide with the mold gate causing scoring of the sealing surfaces on the valve pin and/or the mold gate. Damage may occur immediately, or alternatively, gradually over many cycles of the valve pin opening and closing the mold gate. This can ultimately result in poor quality parts with blemishes around the mold gate, as well as cause other problems with the molding operation. Furthermore, a damaged valve pin or mold gate can be expensive and time consuming to replace and/or repair.
Various valve pin alignment means that align the tip, or “free end,” of the valve pin have been suggested to address the above-identified problem and normally are an additional piece to be assembled with the nozzle. The valve pin alignment means may be positioned towards a downstream end of the nozzle within the nozzle melt channel. However, a high degree of accuracy is necessary in the manufacture of the hot runner injection molding nozzle body, nozzle tip pieces and valve pin alignment means in order to ensure a proper alignment between the valve pin and mold gate when the nozzle is assembled and mounted in the mold plate. Accordingly, there is little room for error in the manufacturing tolerances between the bore or counterbores of the nozzle body and each of the individual components of the nozzle tip and valve pin alignment means, which increases the time and cost of manufacture. If manufacturing errors exist, the misalignment between the valve pin and the mold gate may not be readily corrected without repairing, re-machining and/or replacing one or more of the nozzle components or possibly replacing the entire nozzle.
Thus, a need exists for an injection molding apparatus that provides improved guiding of the valve pin towards the mold gate and that is simpler and cheaper to manufacture.